The X-ray absorber in broad absorption line quasars

Recent observations of Broad Absorption Line (BAL) quasars demonstrated that the soft X-ray emission of these objects is extremely weak, and convincing evidence for very strong absorption by a high column density (similar to 10(23.5) cm(-2)) was obtained for PG 1411 + 442, even though it is one of the few BAL QSOs strongly detected in soft X-rays. This paper examines the ionization status and geometry of the X-ray absorber by combining the properties of the UV lines with the X-ray continuum absorption. We show that the gas has to have large column densities in ions of major UV absorption lines, such as C IV, N V, O VI, and Ne VIII, in order to have sufficient opacity around 0.2-0.35 keV. The UV absorption lines have to be saturated if the X-ray absorber intersects the line of sight to the UV continuum emission region. A uniformly covering UV and X-ray absorption model can be constructed for PG 1411 + 442, but in some other soft X-ray-detected BAL QSOs, such as PG 1001 + 054, the observed line optical depth is much lower than expected from the X-ray-absorbing material. We propose a scheme in which a substantial fraction of the line of sight to the continuum source may be covered by either an optically thick flow or clouds in a narrow velocity range, but in which the total covering factor of either the whole flow or all clouds is close to unity. The absorber can contribute significantly to the extremely highly ionized emission lines, such as O VI lambda lambda 1032/1037 and Ne VIII lambda lambda 770/780, if it covers a substantial fraction of solid angle and if the density is higher than 10(8) cm(-3). However, it has very little impact on the medium- and low-ionization UV lines such as N V and C IV. The profiles of Ne VIII and O VI lines may be indicators for the kinematics of the X-ray absorber in QSOs. The observed Ne VIII line profiles in QSOs suggest that the velocity of the gas projected onto our line of sight is similar to that seen in the outflows of the UV BALs.